Conventionally, there is known a bonding apparatus configured to bond substrates such as semiconductor wafers by an intermolecular force.
In this kind of bonding apparatus, by pushing down a central portion of an upper substrate with a striker while holding an entire peripheral portion of the upper substrate, the central portion of the upper substrate is brought into contact with a central portion of a lower substrate. Accordingly, the central portions of the upper substrate and the lower substrate are bonded by an intermolecular force, and a bonding region is formed. Then, a so-called bonding wave is generated, whereby the bonding region is expanded toward the peripheral portion of the substrate. Accordingly, the upper substrate and the lower substrate are bonded on their entire surfaces (See, for example, Patent Document 1).
To suppress a deformation of the substrates after they are bonded, it is desirable that the bonding wave is expanded from the central portions of the substrates toward the peripheral portions thereof uniformly, that is, in a concentric shape.    Patent Document 1: Japanese Patent Laid-open Publication No. 2015-095579
The bonding wave, however, is not actually expanded in the concentric shape but in a non-uniform manner. This is deemed to be because there is anisotropy in a physical property of a substrate, such as Young's modulus or Poisson's ratio, and a velocity of the bonding wave in a certain crystal direction gets faster or slower than a velocity of the bonding wave in the other crystal direction by being affected by such anisotropy.
Furthermore, it is also deemed to be because, though the bonding wave is expanded in the concentric shape, a deformation amount of the substrate caused by a stress applied thereto is different depending on directions due to the anisotropy of Poisson's ratio or Young's modulus of the substrate.